Alcohol dependence affects 18-20 million Americans and costs our society over $185 billion annually. Treatment for this disorder in not universally accessible and those treatments that exist are not universally effective. The opioid antagonist, naltrexone, approved by the FDA more than 10 years ago, is one of the most efficacious medication treatments for alcohol dependence, but is not widely used and does not work for everyone. Reasons for its lack of widespread acceptance include its low to moderate improvement over placebo and incomplete knowledge of its mechanism of action. Recent advances in pharmacogenetics and neuroimaging have provided new tools to investigate these issues. For instance, in the COMBINE Study we have recently confirmed that a relatively common genetic variant (Asp40) in a mu opioid receptor (OPRM1) may be associated with a higher treatment response to naltrexone. In addition, using advanced functional magnetic resonance brain imaging paradigms, our group has recently shown that naltrexone can blunt the alcohol cue-induced activation in the ventral striatum-nucleus accumbens and in medial prefrontal cortex, of actively-drinking non-treatment seeking alcoholics. We have also discovered that similar alcohol cue-induced activation of the medial prefrontal area is associated with subsequent heavy drinking during treatment of alcohol dependent individuals. The goal of this proposal is to examine in a prospective controlled fashion whether the Asp40 variant of the OPRM1 receptor will predict naltrexone response. In addition, we will examine whether naltrexone might reduce/dampen alcohol cue-induced activation of the ventral-striatum and medial prefrontal cortex and explore whether this dampening might be associated with treatment response. Finally, we will evaluate whether Asp40 individuals will have more naltrexone dampening of alcohol cue- induced brain activation and if this will mediate the medication by genotype interaction observed during treatment. To that end, 320 alcohol dependent individuals will be screened and 160 randomized into a 16-week clinical trial. After OPRM1 genotyping, 80 individuals with at least one copy of the Asp40 allele and 80 individuals who are homogenous for Asn40 will be randomized to naltrexone or placebo, forming a 2 medication (naltrexone or placebo) by 2 OPRM1 genotype (Asp40 by Asn40) design. All subjects will undergo alcohol cue-induced fMRI brain imaging prior to treatment randomization and again between days 7-14 of treatment and will be evaluated over 16 weeks (and during follow-up at weeks 28 and 40) for drinking and other salient outcome variables. Main outcome variables will be differences in percent heavy drinking days and clinical global outcome. Results will assist treatment providers in deciding who might better respond to naltrexone (and by extension other future opioid antagonists) and by elucidating by what mechanism (i.e., reduced alcohol cue salience) this and other similar medications might work. As such, improvement in the treatment of individuals with alcohol dependence could be greatly enhanced.